Not Applicable.
Not Applicable.
The present invention relates to ultrasonic transducers and to methods and articles for coupling ultrasonic transducers to structures such as the wall of a conduit or tank so that the transducer effectively transmits an ultrasonic signal into and generally through the wall to perform various kinds of measurement or interrogation of a fluid contained by the conduit or tank. This field of industrial instrumentation is highly developed, and various forms of transducers as well as a number of different structures for coupling those transducers to a solid wall have been developed. One typical ultrasonic interrogation procedure is to transmit a short ultrasonic pulse bidirectionally across a fluid along a diagonal path having some component in the direction of fluid flow, and to detect changes in transit time indicative of flow velocity. Other procedures may employ continuous waves, or may process scattered energy to detect a frequency change or other parameter of interest. Often, a physical parameter such as temperature or fluid density may be determined by processing the received signals.
The measurement apparatus may involve permanently mounted transducers that thread into tapped openings in the wall, or into specially machined receiving sockets that, in turn, thread into the wall or are welded thereto. Often, however, for reasons of economy, convenience or even necessity, the ultrasonic transducer or signal assemblies are simply clamped in a desired position against the wall to perform a measurement. In that case, care must be taken to assure that there is effective ultrasonic coupling between the transducer and the conduit wall, so that an undistorted signal of sufficient strength is effectively coupled into the wall, and typically through the wall and into the adjacent fluid when the flow or other characteristics of the adjacent fluid are to be measured.
For such clamp-on arrangements, often a strap, chain or U-bolt is used to hold the transducer assembly, or to hold a mounting block that secures the transducer, firmly down against the pipe wall. For certain high temperature applications, the transducer mounts on a standoff or buffer rod assembly, and the proximal face of this buffer must then be secured in effective acoustic contact against the wall. In such a case, typically the transducer is fabricated in, or threaded into, a fitting at one end of the buffer rod opposed to the other end which is to be clamped in contact with the wall. With such clamp-on arrangements, if the wall has an irregular, rough or curved surface, some additional machining preparation, or some further means of enhancing or effecting acoustic coupling must be provided to assure effective coupling.
Many materials have been proposed for achieving such coupling, both in the area of nondestructive testing (where a transducer is typically urged by hand against the wall or metal surface), and in the field of ultrasonic fluid interrogation where clamping elements are often required to fix the transducer at a precisely specified location, often for an extended time.
Among the coupling agents proposed in the past may be found molten glass film, neoprene sheet, Teflon tape, silver-platinum pastes, a gold, platinum, aluminum or zinc foil, and other materials. An extensive listing of known approaches to the coupling of ultrasonic signals appears in the book Ultrasonic Measurements for Process Control by Lawrence C. Lynnworth, Academic Press, (1989) pp. 156-157.
Often before applying one of the listed couplings, the pipe is prepared in some way, for example, by smoothing its surface to minimize the deviations between the wall surface and the buffer or transducer face with which it is to couple. Thus, sanding with abrasive sanding tape may be useful to flatten any peaks or surface protuberances on the conduit, or reduce the height of asperities that would otherwise impair acoustic contact. Foils have sometimes been used to achieve coupling between opposed iron, steel or stainless steel surfaces, and this is effective if the surface finish is adequately smooth. For some applications, synthetic lubricants, anti-seize greases, or thicker or deformably compliant materials of suitable acoustic impedance may be necessary to achieve coupling in the presence of higher surface roughness. The clamping or screw-down pressures necessary for effective coupling may, in some cases, be quite high, about one thousand to several thousand psi, and when a thicker material is employed to effect coupling, attention must also be paid to its strength and modulus, in order to avoid creep, flow or cracking. This is especially true for applications in higher temperature plant conditions, where clamp-on instrumentation is often required.
Often, clamp-on application is required for instrumentation installed after the original plant was laid out, either because awkward conduit positioning, or the inability to shut down or remove a portion of the flow line for machining, prevent non-clamp-on modes of instrument mounting. The clamp-on transducer assembly may then be installed relatively permanently e.g., over ten years, or for extended periods of time such as one to two years between shutdowns for maintenance, so that the long term structural or chemical effects of the coupling and clamping assembly must be considered. These long periods may potentially rule out the use of unstable, reactive or unpredictable coupling materials. In application areas such as nuclear power plant engineering, the potential for, or inability to predict, factors such as pressure induced recrystallization, contact alloying, intergranular corrosion by diffusion of a coupling metal whose melting point is too close to the wall temperature, or other fault growth mechanism in the conduit wall that occur as a result of the material of the coupling may be sufficient to preclude use of clamp-on mounting, or to rule out the application of particular coupling structures. For example, zinc, with a melting point of 419xc2x0 C. may raise problems if clamped to a six millimeter thick conduit of carbon steel or stainless steel carrying fluid of about sixty bar at elevated temperature near 260xc2x0 C.
Because of these factors, the suitability of a coupling mechanism, either in terms of its ultrasonic coupling performance or its physical effects on the wall with which it is in contact, may be questioned.
Accordingly it is desirable to provide an ultrasonic coupler for dependably mounting a transducer or buffer assembly on a wall or conduit.
It is also desirable to provide an ultrasonic coupling assembly that provides effective coupling, is convenient to install, can be used on a wide variety of conduit wall thicknesses and materials, and resists adverse chemical or physical deterioration.
It is also desirable to provide an ultrasonic coupling assembly compatible with diverse transducers, with different mounting mechanisms, and with a range of conduit or tank wall materials.
One or more of the above features and other desirable objects are obtained in accordance with the present invention by a coupling assembly having two or more thin metallic layers. A first layer formed of a gold foil or similar inert substance rests against the wall of a conduit or vessel to form a barrier, while a second layer formed of material such as zinc sheet extends over the first layer to effect ultrasonic coupling with the wall through the barrier. In some embodiments, a third layer covers the second layer and may, together with the first layer, form a symmetrical sheath or envelope sandwiching or entirely enclosing the second layer. In one preferred embodiment the third layer is formed of the same material as the first layer, and is provided by folding the first layer over and on top of the second layer thus also covering at least one edge thereof. An exemplary embodiment is implemented using gold foil approximately one mil thick for the first layer and zinc foil approximately four mils thick for the second layer. The use of inert top and bottom, or outside, layers prevents the inadvertent installation of the coupling element with its second layer contacting the wall. The layered structure may be shaped like the contact surface, as a rectangular or disk-shaped pad or stack. Disk embodiments may employ a stack of separate disks formed of foil or sheet of the respective barrier and coupling materials, to be placed over a coupling face. When used for cryogenic applications, the second layer may employ a material such as aluminum or indium (or alloys thereof) without risk of alloying or adverse metallurgical effects such as grain migration through the first layer. In general, the provision of the first foil layer serves as a barrier against migration from the second layer and thus guards against structural failure of the conduit wall due to metallurgical changes when high temperature/high clamping pressure conditions prevail.